1. Field of the Invention
Embodiments of the present invention generally relate to a solar cell sheet manufacturing apparatus and methods of forming the same.
2. Description of the Related Art
Photovoltaics (PV) or solar cells are devices which convert sunlight into direct current (DC) electrical power. A typical PV cell includes a p type silicon wafer, substrate or sheet typically less than about 0.3 mm thick with a thin layer of n-type silicon on top of a p-type region formed in a substrate. The generated voltage, or photo-voltage, and generated current by the photovoltaic device are dependent on the material properties of the p-n junction and the surface area of the device. When exposed to sunlight (consisting of energy from photons), the p-n junction of the PV cell generates pairs of free electrons and holes. The electric field formed across the depletion region of p-n junction separates the free electrons and holes, creating a voltage. A circuit from n-side to p-side allows the flow of electrons when the PV cell is connected to an electrical load. Electrical power is the product of the voltage times the current generated as the electrons and holes move through an external load and eventually recombine. Solar cells generate a specific amount of power and cells are tiled into modules sized to deliver the desired amount of system power. Solar modules are created by connecting a number of solar cells and are then joined into panels with specific frames and connectors.
The photovoltaic (PV) market has experienced growth with annual growth rates exceeding above 30% for the last ten years. Some articles have suggested that solar cell power production world wide may exceed 10 GWp in the near future. It has been estimated that more than 95% of all photovoltaic modules are silicon wafer based. The high market growth rate in combination with the need to substantially reduce solar electricity costs has resulted in a number of serious challenges for silicon wafer production development for photovoltaics. The amount of solar grade silicon needed to produce solar cells now exceeds the amount of silicon needed by the semiconductor industry.
In general, silicon substrate based solar energy technology follows two main strategies to reduce the costs of solar electricity by use of PV solar cells. One approach is increasing the conversion efficiency of single junction devices (i.e., power output per unit area) and the other is lowering costs associated with manufacturing the solar cells. Since the effective cost reduction due to conversion efficiency is limited by fundamental thermodynamic and physical limits depending on the number of cascaded junctions, the amount of possible gain depends on basic technological advances. Therefore, conversion efficiency improvements are limited making it hard to reach the cost of ownership (CoO) targets. Therefore, one major component in making commercially viable solar cells lies in reducing the manufacturing costs required to form the solar cells.
In order to meet these challenges, the following solar cell processing requirements generally need to be met: 1) the consumption of silicon must be reduced (e.g., thinner substrates, reduction manufacturing waste), 2) the cost of ownership (CoO) for substrate fabrication equipment needs to be improved (e.g., high system throughput, high machine up-time, inexpensive machines, inexpensive consumable costs), 3) the substrate size needs to be increased (e.g., reduce processing per Wp) and 4) the quality of the silicon substrates needs to be sufficient to produce highly efficient solar cells. There are a number of solar cell silicon substrate, or solar cell wafer, manufacturing technologies that are under development to meet the requirement of low silicon consumption in combination with a low CoO. Due to the pressure to reduce manufacturing costs and due to the reduced demands on substrate characteristics, such as surface morphology, contamination, and thickness variation, a number of dedicated substrate manufacturing lines specifically designed to produce solar cells have been established. In these respects solar cell substrates differ in many respects to typical semiconductor wafers.
Crystalline silicon is the material from which the vast majority of all solar cells are currently manufactured. In principle, the most promising substrate manufacturing technologies are the ones where liquid silicon is directly crystallized in the form of a silicon substrate or ribbon (so-called ribbon technologies). Monocrystalline and polycrystalline silicon form the two principle variants of the silicon material used for solar cells. While monocrystalline silicon is usually pulled as a single crystal from a silicon melt using the Czochralski (CZ) process, there are a number of production processes for polycrystalline silicon. Typical polycrystalline silicon processes are block-crystallization processes, in which the silicon substrates are obtained by forming and sawing a solid polycrystalline silicon block, film-drawing processes, in which the substrates are drawn or cast in their final thickness as a silicon film is pulled from a molten material, and sintering processes in which the substrates are formed by melting a silicon powder. Examples of these substrate fabrication process are the EFG process (Edge-defined Film-fed Growth)(e.g., U.S. Pat. No. 5,106,763), the RGS (Ribbon Growth on Substrate) process (e.g., U.S. Pat. No. 4,670,096, U.S. Pat. No. 5,298,109, DE 4,105,910 A1) and the SSP ribbon process (Silicon Sheets from Powder)(e.g., U.S. Pat. No. 5,336,335, U.S. Pat. No. 5,496,446, U.S. Pat. No. 6,111,191, and U.S. Pat. No. 6,207,891). For high speed ribbon type silicon substrate forming processes to be viable the challenge is to reach sufficient substrate quality and solar cell efficiency to provide low cost solar electricity.
Therefore, there is a need to cost effectively form and manufacture silicon sheets for solar cell applications.